The present invention relates to metal ion salts which can be used in electrolytes for producing electrochemical devices, including both primary and secondary batteries, photoelectrochemical cells and electrochromic displays. The salts have a low energy of dissociation and may be dissolved in a suitable polymer to produce a polymer solid electrolyte or in a polar aprotic liquid solvent to produce a liquid electrolyte. The anion of the salts may be covalently attached to polymer backbones to produce polymer solid electrolytes with exclusive cation conductivity.
The ion conductivity of electrolytes is related to the ability of the anion and the cation to dissociate. A low level of ionic dissociation leads to extensive ion pairing and ion clustering and lower conductivity. This effect is most pronounced in polymer electrolytes, because polymers have lower dielectric constants and lower degree of ion complexation than polar aprotic liquid solvents typically used to produce liquid organic electrolytes.
In addition to facile ionic dissociation, the electrolyte must have a high degree of thermal, chemical and electrochemical stability.
Lithium salts that have been used to produce electrolytes for electrochemical devices have generally been selected from LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6, LiPF.sub.6 and LiSO.sub.2 CF.sub.3. Many of these salts are unstable or produce polymer electrolytes with relatively low conductivity.
U.S. Pat. No. 5,162,177 and U.S. Pat. No. 4,505,997 describe a new class of lithium imide salts of the structure LiN(SO.sub.2 CF.sub.3).sub.2. The delocalized anionic charge facilitates dissociation of the ion pair leading to high ionic conductivity both in liquid and polymer solid media. L. A. Dominey in Extended Abstracts of the Annual Automotive Technology Development Contractors' Coordination Meeting, Vol. 2, Dearborn, Mich., Nov. 2-5, 1992, describes a methide analogue lithium salt with the composition LiC(SO.sub.2 CF.sub.3).sub.3 with similar properties to the imide salts.
None of the above described salts allow covalent attachment to polymer backbones to produce polymeric ion conductors with exclusive cationic conductivity. Exclusive cationic conductivity is advantageous for electrolytes in electrochemical devices, such as batteries, as the deleterious opposing voltages produced by the countermoving anionic charges are thereby eliminated. This leads to higher currents and higher power of the devices. In contrast to the imide and methide salts described above, the lithium salts of the present invention allow covalent attachment to polymer backbones.